1. Field of the Invention
The present invention relates to a display device equipped with a light-emitting element or, an electro-optical element in a pixel. In particular, the present invention relates to a display device having a layer including an organic material, a fluorescent material, or a phosphorescent material in the light-emitting element.
2. Description of the Related Art
A display device having a light-emitting element which includes a layer of an organic material between a pair of electrodes and emits light when a current is supplied between the electrodes has been developed. Such a display device has an advantage in reducing thickness and weight, has high visibility due to the self-luminance, and has high response speed. In addition, since power consumption of such a display device may potentially be made very small, it has been actively developed as a device of next generation, and some of such devices have been put into practical use.
In the display device using the light-emitting element having the aforementioned configuration, high image quality and widening of a color gamut have been expected. For example, a display device which can reproduce and display accurate colors in editing in a printing operation, seeing and listening to a work of art, movies, or the like, and catching a real color exactly in telemedicine, has been strongly expected. In view of this, in order to improve a color gamut which can be viewed by human eyes, research to optimize a structure such as improvement of color purity and widening of a color gamut has been carried out (for example, see Reference 1: Published Japanese translation of PCT International Publication for Patent Application No. 2001-039554).
However, there is still a surplus in a color gamut which can be viewed by human eyes, and thus, a color reproduction area of a display device so far is still insufficient. FIG. 39 shows the CIE-XY chromaticity diagram which is established by COMMISSION INTERNATIONALE DE L'ECLAIRAGE (INTERNATIONAL COMMISSION ON ILLUMINATION: CIE) managing standards of color internationally. In an outer boundary of the diagram, a point which is near the rightmost end corresponds to an emission spectrum of 700 nm of red monochromatic light; a point which is near the uppermost end corresponds to an emission spectrum of 546.1 nm of green monochromatic light; and a point which is near the lowermost end corresponds to an emission spectrum of 435.8 nm of blue monochromatic light. In this chromaticity diagram, brightness (chroma) is lower in the inner side since the outer boundary of the graph (a visible area) corresponds to an emission spectrum of monochromatic light while the inner side thereof corresponds to a combination color obtained by combining different kinds of monochromatic light. In the case of expressing a color by an additive color mixture, a plurality of standard colors can reproduce only a color which lies in a position surrounded with a polygon formed of points which are shown in the CIE-XY chromaticity diagram.
When red (R) is shown by the CIE-XY chromaticity diagram, human eyes can perceive a color which has a coordinate near a right region of the chromaticity diagram (a region surrounded with the circumference of the chromaticity diagram and a dotted line 3901 in FIG. 39) as red. In addition, when green (G) is shown by the CIE-XY chromaticity diagram, human eyes can perceive a color which has a coordinate near an upper region of the chromaticity diagram (a region surrounded with the circumference of the chromaticity diagram and a dotted line 3902 in FIG. 39) as green. When blue (B) is shown by the CIE-XY chromaticity diagram, human eyes can perceive a color which has a coordinate near a lower region of the chromaticity diagram (a region surrounded with the circumference of the chromaticity diagram and dotted lines 3903 and 3904 in FIG. 39) as blue. As a specific example, a hi-vision (high definition television broadcasting; HDTV) standard can be given, which has chromaticity coordinates of R (x=0.67, y=0.33), G (x=0.21, y=0.71), and B (x=0.14, y=0.08) (a triangle 3905 in FIG. 39).
According to the method disclosed in Reference 1, a color reproduction area can be expanded in directions of arrows in FIG. 39 by increasing color purity, and brightness of colors recognized by human eyes can be increased. However, there is still a surplus in a color gamut which can be viewed by human eyes. Therefore, it is an essential task to expand the color reproduction area by satisfying the surplus in the color gamut which can be viewed by human eyes.